An avalanche photodiode that selectively amplifies the response of electrons while suppressing the response of holes is described in U.S. Pat. No. 7,432,537, the disclosure of which is hereby incorporated by reference in its entirety.
Photoreceivers often incorporate an avalanche photodiode (APD) structure. A photodiode is a semiconductor device that transforms light into electrical current. The electrical current carried by charge carriers generated by the absorption of light inside the photodiode is called “photocurrent,” and the ratio of photocurrent in Amperes to the incident optical power in Watts is called the photodiode's “responsivity.” An APD is a photodiode with increased responsivity due to internal amplification of the photocurrent by an impact-ionization process. APDs are used for the detection of weak optical signals in situations where their high responsivity boosts the photocurrent signal relative to the downstream circuit noise sources in the detection system.
An APD functions similarly to its older vacuum-tube equivalent, the photomultiplier tube. Light strikes an absorption region and promotes electrons over the relatively narrow band gap of a semiconductor material, creating electron-hole pairs. A photon-stimulated electron is thereby injected into the multiplication region, and is accelerated by a biasing electric field. While many such electrons suffer phonon collisions with the crystal lattice that limit their drift velocity, some accumulate enough energy to boost a plurality of electrons over the ionization threshold level in the multiplication region, creating additional electron-hole pairs to contribute to the photocurrent. This process, known as impact-ionization, is repeated several times, resulting in photocurrent growth as the new electrons and holes accumulate energy and excite additional pairs. However, this benefit comes at the expense of an increase in signal noise due to the fluctuations in the gain, which leads to uncertainty in the amplitude and energy of the incoming signal.
The noise of the multiplication process is determined by the variations in the magnitude and the times of the ionization processes in the multiplication region. The noise of the avalanche process is determined by the variance of the gain distribution about the mean gain level. A narrow distribution of gain about the mean is generally a result of gain attributed to single carrier ionization, whereas a wider variance signifies a more random gain distribution, and is generally a result of two carrier ionization processes.
At the onset of the avalanche, if the carrier type that is injected from the absorption region has an equal or greater than ionization rate as the non-injected carrier type, the greatest contribution to the accumulated gain results from ionization of the injected carrier type, because carriers of that type travel the entire length of the multiplication region. For convenience, we refer to the time that the avalanche initiating carrier transits the entire length of the multiplication region as the primary-carrier transit time. The motion of the carriers within the APD induces current in the circuits external to the APD.
The fastest detectable signal also comes from the injected carrier, as during the initial primary-carrier transit time the generations of progeny carriers traveling with the initiating photoelectron ionize and create progressively increasing numbers of electron-hole pairs as they get closer to the exit point.
During the primary-carrier transit time, the greatest number of non-injected carriers are created at the farthest point from which they may exit, and they then travel the full length of the multiplication stage, creating successive generations of progeny, before entering the absorption region, where they continue to drift before exiting the junction. We can refer to the time that the non-avalanche-initiating carrier transits the entire length of the multiplication region as the secondary-carrier transit time. Ignoring the time of carrier drift through the absorption region, the maximum temporal duration of the signal current contribution due to the ionization of the initiating photoelectron is the primary-carrier transit time plus the secondary-carrier transit time.
The current induced in an external circuit is proportional to the length of the junction, the number of carriers in the junction and the carrier velocities. Assuming no subsequent occurrences of ionization, the magnitude of the current induced by an electron-hole pair created by an ionization event is proportional to the velocities of each carrier and is inversely proportional to the width of the junction. The duration of the current contribution by the electron created by an ionization event is inversely proportional to the electron's velocity and directly proportional to the distance the electron travels from the point of its creation to the cathode. The duration of the current contribution by the hole created by an ionization event is inversely proportional to the hole's velocity and directly proportional to the distance the hole travels from the point of its creation to the anode.
Generally, for a given APD structure with an avalanche multiplication consisting of only single carrier ionization, at higher bias, there is an increase in signal amplitude, but the duration of the impulse response does not substantially increase. The increased probability of ionization of the initiating carrier type that results from the stronger electric field increases the number of ionization chain branches only during the primary-carrier transit time. At that time, the primary carriers of the electron-hole pairs leave the multiplication region and recombine at the contact; the carriers of the other type drift back toward the absorption region, and continue to contribute to the signal current until they have traversed both the multiplication region and the absorption region.
Since enhancement of one carrier's ionization and suppression of the other may not be complete in an APD, assuming a higher ionization rate for the avalanche-initiating carrier over that of the other carrier, there may be an initial signal peak due to the creation of electron-hole pairs by ionization of the injected carrier and its same type progeny near the completion of the initial primary-carrier transit time, followed by a delayed and noisier tail created by the branches of the ionization chains triggered by ionization of carriers of the non-injected type.
For APDs with two-carrier ionization, the duration of the impulse response increases at higher bias. The higher ionization rates caused by the stronger electric field cause both an increased number of branches in the ionization chain and extend the duration of the ionization branches to periods longer than a single carrier transit time.